Time course studies on phosphate transfer in frog urinary bladder

Water handling in Caco-2 cells: effects of acidification of the medium

Caco-2 cells were cultured on permeable supports. At confluence the minute-by-minute net water movement (Jw) was automatically recorded. Simultaneously, unidirectional [14C]mannitol, 22Na+, and/or 36Cl- fluxes and transepithelial resistances were measured. The water and mannitol permeabilities went progressively down between 9 and 16 days after seeding and then stabilized. In this last condition the hydrostatic permeability coefficient (Phydr) was 2.67 +/- 0.31 cm s-1 while the osmotic permeability coefficient (Posm) was 0.0017 +/- 0.0004 cm s-1. Phydr but not Posm was dependent on the temperature and on the presence of Na+ in the medium. A net secretory Jw was observed 16 days after seeding, in the absence of any osmotic, hydrostatic or chemical gradient. This secretory Jw was associated with net Cl- (1.43 +/- 0.43 muequiv h-1 cm-2) and Na+ (1.05 +/- 0.35 muequiv h-1 cm-2) secretions. Amiloride reduced, in open-circuit conditions, both Na+ and Cl- apical to basal fluxes, thus enhancing the net Na+ and Cl- exit. Acidification of the medium (pH 6.2) reversibly increased water and mannitol permeabilities in 10-day-old cultures. In 16-day-old cultures the same shift in medium pH did not change mannitol permeability, while stimulating water secretion. These results, obtained in the absence of supracellular structures (villae, crypts) and subepithelial components (muscular, vascular and conjunctive tissues) indicate that paracellular and transport-associated water pathways are sensitive to changes in the pH of the medium in Caco-2 cell layers.

Increased glucose transfer in the rat jejunum after dietary potassium loading: effect of amiloride

The glucose transfer across the jejunum was measured in Wistar rats under a high potassium diet (HKD). In 12 of 27 HKD animals the transfer coefficient for D-glucose was not significantly higher than in control ones, (7.38 +/- 0.88).10(-5) s-1. In the other 15 a clear increase in glucose transfer was observed, (23.31 +/- 2.50).10(-5) s-1. The D-glucose transfer in the first group (n = 12) was, as in the case of the control rats, insensitive to amiloride section (10(-4) M), while D-glucose transfer became sensitive to amiloride in the second group (mean inhibition 94 +/- 8%, n = 14). A smaller but significant increase in L-glucose and sucrose transfers was also observed when the D-glucose movement was increased. No differences in short-circuit current, transepithelial potential, resistance and mucosa to serosa Na+ fluxes were observed between control and HKD rats and no effects of amiloride (10(-4) M) on these parameters were observed either in control or in HKD animals. [3H]Glucose uptake as also performed in brush-border vesicles prepared from rat jejunum, under control and HKD conditions. The specific and Na(+)-dependent 'overshoot' in D-glucose concentration, in vesicles prepared from HKD rats, became sensitive to amiloride action (10(-5) M). It is concluded that, besides the cellular adaptation induced in the distal portion of the nephron and large intestine, dietary potassium loading induces important modifications in glucose transfer in the rat jejunum.

Water handling in the human distal colon in vitro: role of Na+, Cl- and HCO3-

The minute by minute net water movement (Jw) was measured, in the human distal colon in vitro, simultaneously with the transepithelial potential difference (PD) and short circuit current (SCC) with the following results: (1) An absorptive Jw (+0.36 +/- 0.04 microliters/(min.cm2)) was observed, in 21 cases, when the colon was mounted between two identical standard salines (Na+ 140, Cl- 110, HCO3- 25 mequiv./L) and in the presence of a hydrostatic pressure gradient (delta P) of 13 cm of H2O (mucosal side positive). (2) This absorptive Jw was a linear function of the applied delta P or the imposed osmotic transepithelial gradient (Phydr = 0.22 +/- 0.03 cm/s; Posm = 0.0020 +/- 0.005 cm/s; n = 6). (3) A fraction of this Jw was independent of the presence of any hydrostatic, osmotic or chemical gradient while associated with a serosal side positive and partially amiloride sensitive PD (11.3 +/- 1.8 mV). (4) Both Jw and PD were dependent on the presence of Na+ in the incubating media. (5) Replacement of Cl- by SO(4)2- did not change the absorptive Jw, but increased the observed PD and the transepithelial resistance. (6) HCO3- removal strongly reduced the SCC and PD together with an important increase in Jw. Unexpectedly, other 9 colon fragments spontaneously showed a secretory Jw when mounted between two identical standard salines (-0.55 +/- 0.11 microliters/(min.cm2). In these experiments it was observed that: (7) The tissue moved water against the imposed delta P (13 cm of H2O), while the associated PD (+11.9 +/- 2.1 mV) was similar to the one observed in absorptive fragments. (8) As in the case of absorptive preparations, PD, SCC and the transport associated Jw fell to zero in the absence of Na+. (9) When SO(4)2- replaced Cl-, secretory Jw reversed to absorptive Jw, together with an increase in PD and resistance. In both absorptive and secretory preparations it was finally observed that: (10) norepinephrine (5 x 10(-6) M) decreased SCC and increased the absorptive Jw in a tightly parallel manner (half-times for each response: SCC = 11.4 +/- 2.1 min; Jw = 11.4 +/- 2.0 min, n = 4) and (11) 8-Br cyclic AMP (10(-3) M) increased SCC while simultaneously decreasing the absorptive Jw. It is concluded that the observed Jw in the distal human colon in vitro results from the complex addition of osmotic, hydrostatic and transport associated driving forces. The transport-associated Jw has absorptive and secretory components.(ABSTRACT TRUNCATED AT 400 WORDS)